1. Field of the Invention
The present invention relates generally to a mounting structure for mounting a power unit on the body of a motor vehicle, and more particularly to an electrically controlled mounting structure for the above purpose, capable of providing controlled vibration damping and isolating characteristics, depending upon the conditions of the motor vehicle and the characteristics of input vibrations.
2. Discussion of the Prior Art
In the art of installing a power unit including an engine on the body of a motor vehicle, mounting units are disposed between the power unit and the vehicle body, for example, for isolating or insulating operating vibrations of the engine from the vehicle body, and isolating vibrations of the vehicle body due to bumpy road surfaces, to the engine. Further, such mounting units are used to damp or attenuate the vibrations of the vehicle body and engine, as well as isolating the vibrations from the body and engine, in order to assure improved driving comfort and maneuverability of the vehicle.
Such a power unit mounting structure for a vehicle is usually required to exhibit relatively high vibration damping capability for vibrations having comparatively low frequencies, and relatively high vibration isolating capability for vibrations having comparatively high frequencies. To this end, a fluid-filled mounting structure has been recently proposed as disclosed in Laid-Open Publications Nos. 55-107142 (published in 1980), 55-107145 (published in 1980) and 57-9340 (published in 1982) of Japanese Patent Applications. According to these publications, the fluid-filled mounting structure has an elastic member made of a rubber material, and two fluid chambers which are filled with an incompressible fluid and which commmunicate with each other through an orifice. Further, a movable member (in the form of a plate or diaphragm) is disposed between the two fluid chambers, as a movable partition separating the two chambers for each other, such that the movable member is movable by a predetermined small distance in the direction of input of the vibrations. When the mounting structure receives vibrations of a low frequency, the fluid is forced to flow through the orifice between the two chambers, so that the input vibrations are damped by a restricted flow of the fluid through the orifice. On the other hand, where high-frequency vibrations are applied to the mounting structure, the movable plate is oscillated or reciprocated following the input vibrations acting on the fluid chambers, whereby the pressure pulsations of the fluid within the chambers are absorbed by the oscillating movements of the movable member. Thus, the mounting structure provides excellent vibration-damping characteristics based on a resistance to a flow of the incompressible fluid through the orifice, for the low-frequency vibrations. At the same time, the mounting structure provides improved vibration-isolating characteristics based on oscillating actions of the movable member and the elastic deformation of the elastic member, for the high-frequency vibrations.
However, the vibration damping and isolating characteristics required of such a fluid-filled mounting structure do not always correspond to the frequency range of the vibrations applied, as described above. Stated differently, the required damping and isolating characteristics may be reversed with respect to the frequency of the input vibrations, under some conditions of the vehicle. In this case, the mounting structure exhibiting the consistent spring characteristics corresponding to the frequencies of the input vibrations cannot always be suitably employed for attaining the intended vibration damping and isolating functions.
For example, a mounting structure used for mounting a power unit on a transverse F--F vehicle (front-drive, front-engine vehicle with an engine oriented in the transverse direction of the vehicle) is required to exhibit spring characteristics (dynamic spring constant, and damping coefficient) suitable primarily for damping and isolating engine-idling vibrations, engine-shaking vibrations, vehicle-start vibrations, engine-jerk vibrations, engine-cranking vibrations, and vibrations causing booming noises. The engine-shaking, engine-jerk and engine-cranking vibrations which take place while the vehicle is started or running, will be collectively referred to as "engine shakes". The engine shakes, and the idling vibrations which occur while the vehicle is parked with the engine idling, have almost the same frequency and amplitude, but the mounting structure is required to exhibit entirely opposite spring characteristics, i.e., dynamic spring constant or rate (Kd), and damping coefficient (C), and indicated in Table 1. Consequently, if the mounting structure discussed above is used in the environments involving the engine-idling vibrations, engine shakes, and booming noise vibrations, the mounting structure fails to effectively damp the engine-idling vibrations, though the structure exhibits suitable spring characteristics for the engine shakes and booming noise vibrations.
TABLE 1 ______________________________________ Frequency Desired Types of Vibrations (Hz) Amplitude Kd C ______________________________________ Idling Vibrations 5-30 High Low Low Engine Shakes 5-30 High High High Booming Noise 80-200 Low Low Low ______________________________________
In the meantime, there has been proposed an electrically controlled mounting structure wherein the movable member is positively oscillated by suitable drive means, in the direction in which vibrations are applied to the structure. An example of such an electrically controlled mounting structure is disclosed in Laid-Open Publications No. 60-8540 (published in 1985) of Japanese Patent Application. In this type of mounting structure, the spring characteristics to be exhibited for the same frequency range of vibrations can be changed or reversed by adjusting the conditions in which the movable member is actuated by the drive means.
The electrically controlled mounting structure is adapted to have a high dynamic spring constant by oscillating or reciprocating the movable member in phase with the input vibrations, so that a change in the pressure in the pressure-receiving chamber (one of the two chambers which primarily receives the input vibrations) is amplified, more precisely, so that the fluid pressure is further elevated where the input vibrations act on the pressure-receiving chamber so as to raise the pressure therein, or the fluid pressure is further lowered where the input vibrations act to lower the pressure in that chamber. The mounting structure is given a relatively low dynamic spring constant by oscillating the movable member such that the oscillation phase is oposite to the phase of the input vibrations, so that a change in the pressure in the pressure-receiving chamber (one of the two chambers which primarily receives the input vibrations) is reduced, namely, so that the fluid pressure is lowered where the input vibrations act on the pressure-receiving chamber so as to raise the pressure therein, or the fluid pressure is elevated where the input vibrations act to lower the pressure in that chamber.
Further, the damping coefficient of the mounting structure is increased by advancing the oscillation phase of the movable member by 90 degrees relative to the phase of the input vibrations, and is decreased by retarding the oscillation phase of the movable member by 90 degrees relative to the vibration phase.
Described in other words, the electrically controlled mounting structure equipped with the built-in drive means for reciprocating the movable member can be readily controlled so as to provide the different or opposite spring characteristics, i.e., high dynamic spring constant and damping coefficinet, and low dynamic spring constant and damping coefficient, by oscillating the movable member with an advanced phase of 0-90 degrees or a retarded phase of 90-180 degrees, relative to the input vibration phase. Accordingly, this type of mounting structure is capable of exhibiting spring characteristics adapted to different characteristics of vibrations in the same frequency range, which occur as on a transverse F--F vehicle, depending upon the conditions of the vehicle.
Although the electrically controlled mounting structure with drive means discussed above is excellent in the operating principle itself, it is difficult to adjust its vibration damping and isolating capacity of the movable member, since the oscillation or reciprocation stroke of the movable member is constant or fixed. The capacity of the movable member is determined by a sum of the oscillation stroke and a pressure-receiving surface area of the movable member. Since both of these parameters are fixed in the conventional arrangement, the damping and isolating capacity due to the oscillation of the movable member is fixed.
Another drawback of the conventional electrically controlled mounting structure is associated with the arrangement of the drive means for moving the movable member. More particularly, the movable member consists of a substantially circular plate of a magnetic material, while the drive means employs electromagnet means energized for attracting the disc-like movable member by a magnetic force, and thereby reciprocating the movable member. According to this arrangement, the movable member cannot be smoothly oscillated at a high frequency, or the oscillation of the movable member upon energization of the drive means at a high frequency cannot follow high-frequency vibrations received by the mounting structure. This means insufficient capability of the movable member for high-frequency vibrations. Further, the movable member used in the conventional arrangement suffers from uneven distribution of the magnetic force produced by the drive means, which leads to local variations in the force of attracting the movable member, and consequently results in partial attraction of the movable member. This may also cause reduced ability of the movable member to follow high-frequency vibrations, and therefore the conventional electrically controlled mounting structure is not satisfactory in terms of its capability to deal with the high-frequency vibrations, which may cause booming noises, as encounterd on the F--F vehicle of the transverse-engine type.